feat(ui): linear-gradient backgrounds, object-fit and aspect-ratio

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/ludic.ui/attr_fit.ludic

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+# attr_fit.ludic - aspect-ratio (16 / 9, or 1.5) and object-fit (fill, contain, cover, none,
+# scale-down) for pictures and natives, and a list split at its top-level commas
+function el_fit_props(n: UiNode, key: string, s: string) -> bool {
+  if key == "aspect-ratio" {
+    let parts = lss_split(s, 47)
+    var r = el_num(tpl_words(parts[0]))
+    if len(parts) > 1 and el_num(tpl_words(parts[1])) > 0.0 { r = r / el_num(tpl_words(parts[1])) }
+    if tpl_words(s) == "auto" { r = 0.0 }
+    nx(n).ratio = Math.max(r, 0.0)
+  } else if key == "object-fit" {
+    let w = tpl_words(s)
+    var k = 0
+    if w == "contain" { k = 1 } else if w == "cover" { k = 2 } else if w == "none" { k = 3 } else if w == "scale-down" { k = 4 }
+    nx(n).fit_obj = k
+  } else { return false }
+  return true
+}
+# "a, rgba(1, 2, 3, 0.5) 40%, b" -> its three parts
+function el_commas(s: string) -> []string {
+  let out = new []string
+  var at = 0
+  var depth = 0
+  for i in 0 .. len(s) {
+    if s[i] == 40 { depth += 1 }
+    if s[i] == 41 { depth -= 1 }
+    if s[i] == 44 and depth == 0 {
+      push(out, s[at..i])
+      at = i + 1
+    }
+  }
+  push(out, s[at..len(s)])
+  return out
+}